Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. It has been known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the aluminophosphates (ALPO') and the silicoaluminophosphates (SAPO'). SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO is generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminium- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formulation into a SAPO catalyst, and its use in converting a hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, is shown in U.S. Pat. Nos. 4,499,327, 4,677,242, 4,677,243, 4,873,390, 5,095,163, 5,714,662 and 6,166,282, all of which are herein fully incorporated by reference.
It has been discovered that metalloaluminophosphate molecular sieves such as aluminophosphate (ALPO) and especially silicoaluminophosphate (SAPO) molecular sieves, are relatively unstable to moisture containing atmospheres such as ambient air when in the calcined or partially calcined state; this state is sometimes referred to as the activated state. It has also been observed that the relative stability is in part related to the nature of the organic templating agent used in the manufacture of the SAPO molecular sieve. Briend et al., J.Phys. Chem. 1995, 99, 8270-8276, teaches that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data is presented, however, which suggest that over at least the short term, crystallinity loss is reversible. Even over a period of a couple years, the data suggest that crystallinity loss is reversible when certain templates are used.
U.S. Pat. No. 4,681,864 to Edwards et al. discusses the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability. However, stability can be improved by using a particular activation process. According to the process, retained organic template present from the synthesis of the SAPO-37 is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800° C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425° C. The heating process is first carried out in the presence of an oxygen-depleted gas, and then in the presence of an oxidizing gas. The objective of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the molecular sieve.
U.S. Pat. No. 6,051,746 to Sun et. al. discloses a process for the conversion of oxygenated organic materials to olefins using a modified small pore molecular sieve catalyst. The molecular sieve catalyst is modified with polynuclear aromatic heterocyclic compounds in which at least three interconnected ring structures are present having at least one nitrogen atom as a ring substituent, and with each ring having at least five ring members.
European Published Application EP-A2-0,203,005 discusses the use of SAPO-37 molecular sieve in a zeolite catalyst composite as a commercial cracking catalyst. According to the document, if organic template is retained in the SAPO-37 molecular sieve until a catalyst composite containing zeolite and the SAPO-37 molecular sieve is activated during use, and if thereafter the catalyst is maintained under conditions wherein exposure to moisture is minimized, the crystalline structure of the SAPO-37 zeolite composite remains stable.
PCT Publication No. WO 00/74848 to Janssen et al. describes a method of protecting the catalytic activity of silicoaluminophosphate molecular sieves by covering the catalytic sites with a shield prior to contacting with an oxygenate feedstock. The shielding may be achieved by retaining template within the pores of the molecular sieve, by using carbonaceous materials, or by using an anhydrous gas or liquid environment.
PCT Publication No. WO 00/75072 to Fung et. al. discloses a method for addressing the problems relating to protecting molecular sieves from damage due to contact with moisture and damage due to physical contact. The method requires the heat treatment of a molecular sieve containing a template under conditions effective to remove a portion of the template from the microporous structure and cooling the heated molecular sieve to leave an amount of template or degradation product thereof effective to cover catalytic sites within the microporous structure.
PCT Publication No. WO 00/74846 to Janssen et. al. discloses a method for preserving the catalytic activity of silicoaluminophosphate molecular sieves which comprises heating of template-containing silicoaluminophosphate in an oxygen depleted environment under conditions effective to provide an integrated catalyst life which is greater than that obtained using a non-oxygen depleted environment.
U.S. Pat. No. 6,051,745 to Wu et. al. is concerned with overcoming the problem of the excessive production of coke, which occurs when some silicoaluminophosphates are used as catalysts in the conversion of oxygenated hydrocarbons to olefins. The solution proposed is the use of nitrided silicoaluminophosphates. Nitridation is achieved by the reaction of the silicoaluminophosphate with ammonia at elevated temperatures, typically in excess of 700° C. The nitridation reaction is essentially irreversible and destroys irreversibly the acidic sites of the molecular sieve, as the acidic OH groups are converted to NH2 groups during the nitridation process.
U.S. Pat. No. 4,861,938 to Lewis et.al describes a process for converting feedstocks. Matrix material used in the manufacture of the catalyst for the process may be conditioned prior to catalyst manufacture by exposure to ammonia.
U.S. Pat. No. 5,248,647 to Barger describes a process for the hydrothermal treatment of silicoaluminophosphate molecular sieves. The process requires the treatment to be undertaken at temperatures in excess of 700° C. to destroy a large proportion of the acid sites whilst at the same time retaining a significant proportion of the original crystallinity. Also disclosed in this document is a test method for determining the molecular sieve acidity. This test method requires the adsorption of ammonia onto the molecular sieve, followed by desorption within the temperature range of 300 to 600° C. and titration of the desorbed ammonia
As seen from the disclosures described herein, many metalloaluminophosphate molecular sieves will exhibit a shortened catalytic life when exposed to a moisture-containing environment. This loss of catalytic life is, in some instances, irreversible, and can occur over a very short period of time. In essence, this loss of catalytic life is due to a loss in the number of acid catalytic sites. In addition there may be irreversible loss of molecular sieve crystallinity and porosity on ageing during storage and handling after manufacture.
It is desirable therefore to develop methods for the treatment of metalloaluminophosphate molecular sieves and catalysts containing these molecular sieves, which ensure that the catalytic properties and physical properties of these materials, such as porosity and crystallinity, are retained after storage and handling.